Integers: BCD
	Since people think in decimal, it may be convenient to use a decimal-like representation.
	Binary coded decimal (BCD) uses unsigned binary to represent each decimal digit.
	How many bits are needed?
		4 bits per decimal digit: ceil (lg 10) = 4
	Since 4 bits can represent 16 values, 6 values are unused for every digit.
	Advantages
		Decimal input/output doesn't require complicated conversion.
		Fewer problems with repeating fractions.
	Some early computers used BCD representation, and many calculators still use BCD.
	Example:
		Represent 1739ten in BCD:
			decimal	1	7	3	9
			BCD	0001	0111	0011	1001
	Since calculators can be much slower than computers, they can use hardware which
		operates on 4 bits at a time.
	Negative values are represented using 10's complement (similar to 2C).